With further size reduction and sophistication of electronics, there is an increasing demand for high-density mounting of electronic components, e.g., semiconductor chips, on substrates. High-density mounting technologies include a flip chip attach method. According to this method, projection electrodes are formed on a surface of a chip, and the chip is directly connected to the substrate with the surface of the chip faced down. With the flip chip attach method, a semiconductor chip and a substrate are directly connected to each other. Thus, the connection portions may hardly absorb stress caused by a difference in coefficient of linear expansion between the silicon chip and the substrate. In such a case, a crack may be produced in the connection portions. This may result in poor connection reliability. To overcome this problem, a technique is used with which a sealing resin referred to as underfill material is filled between the semiconductor chip and the wiring substrate. The use of underfill material makes for increased reliability of connection against thermal stress such as heat cycles and against physical stress such as impact or bend.
Patent Document 1 discloses an underfill material containing an epoxy resin, a polyfunctional aliphatic epoxy, a basic curing agent, and inorganic filler. Generally, a phenol-based, an acid anhydride-based, and an amine-based curing agents are known as curing agents for epoxy resin. Recently, however, underfill material containing a basic curing agent, e.g., an amine-based curing agent, mixed therein is used in many cases for its high ability to control the glass-transition temperature.
The coefficient of linear expansion of the silicon chip is 4 ppm/° C., and the coefficient of linear expansion of the substrate, e.g., a glass epoxy substrate, is 20 ppm/° C. The filler is mixed in the underfill material to absorb the difference in coefficient of linear expansion. For example, powder of silica, alumina, boron nitride, or aluminum nitride is usually used as the filler.
Meanwhile, recently, copper pillar bumps have come into use particularly in high-density flip chip attach. The copper pillar bumps have, for example, the following advantages: reduced bump pitches as compared with solder bumps used heretofore; minimized environmental impact because of the reduced amount of lead use; a high radiation property because of the high thermal conductivity; and reduced parasitic resistance because of the high electric conductivity.
FIGS. 7(a) to 7(e) are cross-sectional views sequentially illustrating the processes of a method of mounting electronic components by the flip chip attach that has been conducted so far using the copper pillar bumps and underfill. FIG. 7(a) is a cross-sectional view of a wiring substrate 101 with electrode terminals 102 formed on a surface thereof. Resist patterns 103 are formed on this wiring substrate such that the resist patterns 103 fill the spaces between the electrode terminals 102 (FIG. 7(b)). Next, solder bumps 104 are each formed on the electrode terminals 102 (FIG. 7(c)).
Generally, SnPb is used as a material of the solder bumps 104. To address an environmental issue, however, Pb-free SnAgCu is used in some cases. The resist patterns 103 are formed filling up the spaces between the electrode terminals 102. This structure prevents short circuit between the adjacent solder bumps. Next, a bare chip 106 with copper pillars 105 formed on a surface thereof is placed on the wiring substrate 101. The chip substrate 106 and the wiring substrate 101 are aligned to each other. The chip substrate 106 and the wiring substrate 101 are heated at a temperature for joining solder, such that electrical connection is established between the copper pillars 105 and the electrode terminals 102 through the solder bumps 104 (FIG. 7(d)). Next, an underfill material is heated at about 80° C. to turn it into liquid form. The liquid is injected into the spaces between the chip substrate 106 and the wiring substrate 101. Further, the underfill material is heated at about 150° C. to be cured (FIG. 7(e)).
However, when flip chip attach is performed with the copper pillars by using the mounting method as illustrated in FIGS. 7(a) to 7(e), a phenomenon is observed in which the filler contained in the underfill separate in the resin, i.e., the filler is agglomerated in the resin, during heat curing. The phenomenon causes areas with no filler present to be formed in the underfill. Because of this, the difference in coefficient of linear expansion between the silicon chip and the substrate is partially not absorbed, and development of cracks in bumps is brought about.    Patent Document 1: JP-A-2007-56070